Dimmer circuits are commonly used to control power, in particular alternating current (AC) mains power, to a load, such as a light source. In one existing method, a light source can be dimmed using phase controlled dimming whereby power provided to the load is controlled by varying the amount of time that a switch connecting the load to a mains power source is conducting during a cycle of the AC (i.e. varying the duty time). Specifically, AC power to the load is switched ON and OFF during each half cycle of alternating current and the amount of dimming of the load is provided by the amount of ON time in relation to the OFF time for each half cycle.
Phase control dimmer circuits generally operate as trailing edge or leading edge dimmer circuits, and the two circuits are suited to different applications. In leading edge circuits, power is switched OFF at the beginning of each half cycle. In trailing edge circuits, power is switched OFF later in each half cycle (e.g. towards the end of each half cycle). Leading edge dimmer circuits are generally better suited to controlling power to inductive loads, such as small fan motors and iron core low voltage lighting transformers. Trailing edge dimmer circuits, on the other hand, are generally better suited to controlling power to capacitive loads, such as drivers for Light Emitting Diode (LED) lights.
Phase control dimmer circuits, however, can produce line conducted harmonics causing electromagnetic interference (EMI) emissions when switching ON and OFF power to the load—particularly, for instance, switching ON and OFF power to complex loads such as compact fluorescent lighting (CFL) and LED light drivers. More specifically, these dimmer circuits include a switching circuit for controlling delivery of AC power to the load by conducting power to the load in an ON state (conduction period) and not conducting power to the load in an OFF state (non-conduction period). During the OFF state of each half cycle of AC, power is available to the dimmer circuit for operation thereof. This AC (line) power provided to the dimmer circuit in the non-conduction period is first rectified by a rectifier. The rectified dimmer voltage (e.g. rectified via full-wave rectifier) is of a pulse form, normally—under symmetrical non-conduction period operating conditions of the dimmer circuit—having repetition rate equal to twice the line frequency. If asymmetrical conditions exist in the non-conduction periods, then repetition rate becomes line frequency (or lower is possible), which manifests itself as lamp flicker in if the load type is a LED light driver or a CFL driver.
Generally, the input stage of the load (e.g. electronic load such as CFL or LED driver) includes some EMI filtering capacitance on the AC side of a diode bridge rectifier, in addition to a small (or large) bulk capacitance on the DC side. The input stage of the load comprising the EMI filter, bridge rectifier and a bulk reservoir capacitor is, however, largely responsible for producing asymmetrical operating conditions in the non-conduction periods of typical exemplary existing 2-wire trailing edge phase control dimmer circuits which run constant conduction periods. Indeed, when such a load is controlled by the exemplary dimmer circuits running with constant conduction periods at the detection of the dimmer voltage zero-crossing, this load type circuit topology is inherently prone to producing asymmetrical power draw at each half-cycle polarity of the AC due to phase-shift effects on the dimmer voltage zero-crossing caused by at least in part the ripple voltage components. Hence, the asymmetrical power draw at each half-cycle polarity of the AC can result in the undesirable effect of flickering of an LED light driven by the load.
More specifically, with respect to an exemplary 2-wire trailing edge phase control dimmer circuit of the prior art, the instantaneous voltage appearing across the dimmer circuit during the non-conduction part of each AC half-cycle is equal to the instantaneous line voltage subtract the rectified load voltage including any ripple voltage appearing on DC side of the diode bridge rectifier (disregarding the comparatively low diode forward voltages of the diode bridge). The remaining line voltage component applied to the dimmer circuit is thus always lower than the full line voltage and consequently has phase advancement of the zero-crossing by an amount in proportion to the instantaneous magnitude of rectified load voltage appearing on the DC side of the diode bridge rectifier. The dimmer circuit commences each half-cycle conduction period immediately upon detection of the zero-crossing, which now has an amount of phase advancement determined by the instantaneous rectified load voltage and the ripple voltage. Thus, increased phase advancement in one half-cycle polarity results in reduced magnitude of corresponding rectified load voltage. This increase in phase advancement then results in reduced phase advancement for the successive half-cycle conduction period, which further results in increased magnitude of corresponding rectified load voltage, which again produces increased phase advancement of the successive half-cycle conduction period. Accordingly, in this exemplary prior art dimmer circuit, AC power can be continually applied asymmetrically to the load during symmetric conductions periods, which can cause the abovementioned problems such as flicker in lighting loads.